1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Among image forming apparatuses, there is one type of apparatus that charges and writes an image onto an image carrier by rotating the image carrier having a drum shape or a belt shape, thereby forming an electrostatic latent image, adheres a toner to the latent image to visualize the image to form a toner image by a developing device, and then directly transfers the toner image onto a transfer material such as paper and an overhead projector (OHP) film to form the image thereon. There is also an image forming apparatus that indirectly transfers an image onto a transfer material via a belt-shaped intermediate image transfer unit, for example, thereby forming the image on the transfer material.
The latter image forming apparatus primarily transfers an image formed on an image carrier such as a photosensitive unit onto an intermediate transfer unit such as an intermediate transfer belt to form an image thereon using a primary transfer unit such as a primary transfer roller. The apparatus secondarily transfers the image on the intermediate transfer unit onto a transfer material such as paper to form the image, using a secondary transfer unit such as a secondary transfer roller provided in contact with the intermediate transfer unit.
This image forming apparatus includes two types of multicolor image forming apparatuses below.
A first image forming apparatus called a revolver type or a rotary type apparatus sequentially forms images of different colors onto one image carrier, sequentially primarily transfers and superimposes the images onto an intermediate transfer unit to form a multicolor image, and collectively secondarily transfers the images onto a transfer material to form a multicolor image on the transfer material.
A second image forming apparatus called a tandem apparatus has plural imaging stations along an intermediate transfer unit, forms images of different colors onto respective image carriers at these imaging stations, sequentially primarily transfers these images onto an intermediate transfer unit to form a multicolor image, while running the intermediate transfer unit, and collectively secondarily transfers the image onto a transfer material to form a multicolor image, using a secondary transfer unit.
These types of image forming apparatuses used to form multicolor images containing full colors are required to stably achieve image quality including color reproducibility. To meet the above requirement, there is a method of forming a concentration detection pattern image, what is called a patch pattern, on an intermediate transfer unit, optically reading the patch pattern using an optical detector, and feedback controlling various kinds of parameters used for an image forming condition based on a result of the reading, as disclosed in Japanese Patent Application Laid-open No. H10-161388.
In the feedback control, an image concentration sensor as an optical detector measures the amount of a toner forming a patch pattern adhered to an intermediate transfer unit. When a measuring result does not meet a predetermined condition, various kinds of parameters are adjusted to meet this predetermined condition. For example, a writing output characteristic, a charge characteristic of an image carrier, a charge characteristic affecting the adhesiveness of a toner in a developer, and a developing bias characteristic to control the adhesion amount of the toner are adjusted.
A patch pattern is formed larger than a detection range detected by the optical detector, on the intermediate transfer unit. A saturated part of the output from the optical detector, that is, a part where a patch pattern is formed in the whole range detected by the optical detector is measured. The amount of adhesion of the toner is calculated based on a result of the detection. The calculated amount of adhesion of the toner is used to determine predetermined concentration, and is also used to calculate a timing of formation of each color patch pattern. The calculated timing of formation of a patch pattern is used to determine a position of forming each color image.
As a concentration control method according to a measurement of patch pattern concentration, there is a method as disclosed in Japanese Patent Application Laid-open No. 2002-132097. There is also a method disclosed in Japanese Patent No. 2642351 regarding the control of an image position (a positional deviation due to color misregistration) based on a detection of a position of a patch pattern.
A patch pattern is formed in constant concentration in an area not superimposed with a starting end of the next image forming area, separately from the original image forming area. Therefore, when a patch pattern is formed to measure concentration or detect a position, a patch pattern as an image not yet transferred is transferred onto a secondary transfer unit in a secondary transfer nip that brings the secondary transfer unit into contact with an intermediate transfer unit. Consequently, there is a risk that the toner transferred to the secondary transfer unit is adhered to the back surface of a transfer material passing through the secondary transfer nip, resulting in staining of the back surface of the transfer material.
Therefore, to avoid the staining of the back surface of the transfer material, conventionally, when a patch pattern is formed, the secondary transfer unit is separated from the intermediate transfer unit when the patch pattern passes through the secondary transfer nip. In other words, as shown in a flowchart in FIG. 17, an image formation is started at S1. When the timing is determined as a process control timing at S2, the secondary transfer unit is separated at S3 after the secondary transfer of a normal image ends. At S4, an image forming condition of a patch pattern is changed according to need. At S5, a patch pattern is generated. When all patch patterns are detected at S6, a result of detecting the patch patterns is feedback to the control at S7. At S8, the secondary transfer unit is contacted. At S9, when there is further an image forming job, the process returns to S1. When there is no image forming job, the process ends.
When the timing is not the process control timing at S2, the secondary transfer bias is switched to antipolarity to turn in idle in a state that the secondary transfer unit is brought into contact with the intermediate transfer unit between the transfer material and the secondary transfer unit, at S10. With this arrangement, the toner adhered to the secondary transfer unit is adhered to the intermediate transfer unit to execute cleaning. The process proceeds to S9. When there is an image forming job, the process returns to S1. When there is no more image forming job, the process ends. According to this method, when four gradation patterns are used for each color, a pattern layout shown in FIG. 18 is used.
In FIG. 18, reference numeral 1 denotes a belt-shaped intermediate transfer unit, 2 denotes a normal-image forming area, 3 denotes a roller-shaped secondary transfer unit, and 4 denotes an optical detector supported by a supporting member 5. Py1, Py2, Py3, and Py4 denote patch patterns of four gradations of yellow formed on the intermediate transfer unit 1, and Pc1 and Pc2 denote a part of patch patterns of four gradations of cyan formed on the intermediate transfer unit 1.
In detecting a patch pattern, when a configuration for canceling the secondary transfer process is used as described above, and when a secondary transfer roller is used as a secondary transfer unit as disclosed in Japanese Patent Application Laid-open No. 2002-123052, the secondary transfer roller is separated from the intermediate transfer unit. Therefore, it is necessary to set time required for this separation, and a distance of conveying a transfer material to be used a secondary transfer is increased.
Specifically, this operation is carried out at a timing shown in FIG. 19. Therefore, according to the conventional method, an image is formed after the secondary transfer unit is separated to prevent the influence to the image. Consequently, substantial downtime occurs.
In separating the secondary transfer unit to be used for the second transfer, oscillation due to this operation occurs in the intermediate transfer unit. As a result, there occurs a disturbance in an optical relationship with the optical detector at the time of detecting concentration of the patch patterns executed in the canceled state of the second transfer process. In other words, an optical distance is disturbed. This results in an error in the concentration detection.
To avoid the above inconvenience, the process relating to the image formation is once stopped after the image forming process. The roller is separated in this state, and concentration of patch patterns is detected in the state that the process relating to the image formation is started. In this way, the influence of oscillation generated in the intermediate transfer unit can be avoided. According to this method, however, a lapse time due to the stop of the process and restarting increases considerably, and there is a risk of increase in the user waiting time. Particularly, when the optical detector is disposed opposite to the extension part of the intermediate transfer unit as disclosed in Japanese Patent Application Laid-open No. 2002-123052 and Japanese Patent Application Laid-open No. 2003-167394, for example, oscillation of the intermediate transfer unit gives a large influence.
When the secondary transfer roller is used as a secondary transfer unit to solve the inconvenience of staining the back surface of the transfer material, a cleaning device that removes the toner adhered to the secondary transfer roller is also provided. However, in this case, the separate provision of the cleaning device becomes a hindrance to reduction of the space and the size of the image forming apparatus. Accordingly, this hinders cost reduction.
On the other hand, the patch pattern provides information concerning an image of each color, and all images need to be formed before the secondary transfer is started. However, when the secondary transfer is started to shorten the image forming process time, there is a risk that the patch pattern of the final color is not formed. This has the inconvenience that an optimum patch pattern forming condition is hindered by the start of the secondary transfer.